1. Field of Invention
The present invention relates generally to optoelectronics and laser technology, and more specifically to methods employed in industrial and communication applications using a control laser element and laser array source for sensing and stabilizing laser array power and wavelength, without reducing array output power (i.e. lossless), and reducing drift in a wavelength stabilized laser source.
2. Description of the Prior Art
Wavelength stability in the optical output signal of the light source is necessary in many sensor systems and telecommunication systems using optical fibers. Depending upon the system, a high precision of wavelength stability, in parts per million, is required. Similarly, a high degree of stabilization is also necessary in the lasers used in fiber optic telecommunication systems. Further limiting the influence of noise and drift in these sensitive systems would enable additional traffic utilization in these systems.
Communication service providers are experiencing significant consumer demands to accommodate additional bandwidth in optically-based communications systems and the demand is ever-increasing. Today""s optical communication systems and networks field rising consumer demands for e-mail, video, multimedia, data and voice-data transmission requirements across a variety of communication protocols. In the future, all indications are that the use of fiber optic networks will become even more prevalent as a preferred medium for transferring information as the marketplace for wide-band services matures. It is anticipated that additional services such as enhanced pay-per-view, video-on-demand, interactive television and gaming, image networking, video telephony, CATV, and ISDN switching services will be dependent on and be substantial to users of such systems.
Devices representative of existing technology for implementing fiber optic networks, which are known in the industry, include waveguide division multiplexers (WDMs), fiber amplifiers such as erbium doped fiber amplifiers (EDFAs), and add/drop networks. These devices, as well as other components of a fiber optic network, contribute to or are affected by power level variances in the independent channels of a fiber optic network and may also contribute to noise and drift in the system. Therefore, the maturation of economically feasible and technically satisfactory fiber optic networks for the multiple users and diverse uses previously described, is dependent upon the stabilization of power levels in the independent channels of a fiber optic network, the reduction of wavelength spacing between adjacent channels, and the reduction of drift in a stabilized optical source.
Lasers are employed in numerous applications, particularly within fiber optic networks. Often, due to constraints in design or by way of limitations of devices in the system, it is important to stabilize and control the laser output wavelength, i.e. locking the wavelength to a reference wavelength. Similarly, because a single semiconductor laser may not generate sufficient power for the system, high power semiconductor laser sources such as semiconductor laser arrays, generating high optical power, are utilized. These high power laser arrays are made of single stripes of semiconductor lasers and may also be stacked to form two dimensional laser arrays. As is often the practice, each element of a laser array may be of a different wavelength than another element. Controlling and stabilizing the wavelength and output of the arrays, while minimizing losses to or splitting of the output power is critical to system performance.
In particular, in a wavelength division multiplex communication system, there typically exists a high number of independent channels in a transmission line. These channels are independent of each other and are well-suited for multimedia and multi-data transmission and communication. Wavelength division multiplexed (WDM) communication systems have further advanced to dense wavelength division multiplexed (DWDM) systems, the DWDM systems being point-to-point systems designed to increase the capacity of installed fiber. DWDM systems currently provide up to 400 GBps capacities and beyond over a single strand of fiber, and provide transmission capabilities four to eight times those of traditional time division multiplexed (TDM) systems. DWDM systems require 0.1-20 ppm frequency stabilization over their anticipated life spans which are estimated at 25 years.
A DWDM system typically includes at least one optical amplifier having two key elements: an optical fiber that is doped with the element erbium and the amplifier. Typically a laser is employed to energize the erbium with light at a specific wavelength and the erbium thereby acts as a gain medium that amplifies the incoming optical signal. The gain of the optical amplifier is dependent upon the optical power in the signal channel. The strength of the incoming signal is desired to be optimal. If the power of the optical signal is degraded, as may occur due to phase modulations, the signal may be insufficient to meet demands of the communication system, often due to too little power resulting in not enough gain from the amplifier.
In a WDM and DWDM communication systems, many channels can be secured by narrowing the channel gaps and transmitting the signals in the same tunable range for each channel. In order to keep the gaps smaller than a change in width due to drift of wavelength of a semiconductor laser, control is necessary to reduce the influence of the drift of wavelength. In order to reduce the influence of drift of wavelength it is necessary to stabilize the wavelength absolutely or relatively.
The wavelength of light emitted from a laser source varies as a function of the operating temperature, and of the current applied to the energy source for excitation. Controlling the wavelength output in an optical multichannel system with narrow channel spacing is further complicated by the fact that laser output wavelength is influenced by other factors such as acoustic vibrations as an example.
Critical to these systems is the ability to reduce wavelength spacing between adjacent channels, and thereby increase the number of channels available to be utilized within any particular waveband. Employing a method for wavelength stabilization is necessary to compensate for the effects of temperature and current variation to obtain a reasonable degree of stabilization. Employing a method for wavelength stabilization without reducing power of the optical light sources, for a plurality of optical light sources, is also desired. Employing a method which utilizes high power optic sources and has a means for stabilizing the wavelength of the source, without dramatically reducing the output of the source, is especially desired.
A number of light source stabilization approaches are known for application to discrete laser devices. The following three patent applications referred to following are hereby incorporated by reference into the present application. For example, U.S. Pat. No. 4,842,358 discloses optical signal source stabilization using an interferometer forming optical beams through a birefringent crystal interferometer, wherein the beams have similar intensities at a desired source frequency. The difference between the intensities of each beam generates an error signal which is directed to a means for altering the drive current of the source to produce an optical output signal which minimizes the error signal.
U.S. Pat. No. 5,167,444 discloses optical signal source stabilization, wherein the optical output signal from the source is stabilized by adjusting its frequency to maintain a selected optical transmission through a Fabry-Perot interferometer. The interferometer has a xe2x80x9csplit-levelxe2x80x9d gap whereby the gap is split into two discrete portion of different widths. A photodetector associated with each portion generates an electric signal indicative of the beam intensity or power transmitted through that portion of the gap. At a desired wavelength of the optical signal incident in the interferometers, the transmission of the light through the gap will be approximately equal in both gap portions, resulting in substantially equal amplitudes of the electrical signals from the two photodetectors. The two photodetectors"" signals are then fed into a drive current of the optical source so as to minimize the amplitude error signal. This minimum error will occur at the desired wavelength.
U.S. Pat. No. 5,428,700 discloses the stabilization of a wavelength of a light source by introducing a collimated beam from the source into a Fabry-Perot cavity having a continuously decreasing width along at least one axis that is normal to the axis of beam propagation. Photodetectors at the output side of the etalon produce signals which are employed in a feedback loop, whereby the wavelength of the light source is controlled to minimize the amplitude difference between the photodetector output signals.
Similarly, other methods, known in the art, employ variations of the splitting of discrete laser light output whereby laser light may be split prior to an optical fiber, or laser light is split from the back side of the laser, between the discriminator and the photodetector used for power control. Each of the known approaches have certain limitations. U.S. Pat. No. 4,842,358 requires relatively expensive components such as birefringent crystals made to precisely controlled tolerances. U.S. Pat. No. 5,167,444 is relatively difficult to adapt to multiple light source applications, such as arrays. U.S. Pat. No. 5,428,700 splits the power from the laser package output fiber and send a portion of the light to the wavelength discriminator thereby reducing the output of the laser light by 5-10% or more. Splitting laser light from the back side of the laser further reduces an already low power source, as the back side of a typical communications laser is generally 5%-25% of the power of the front side, and may further limit the available output power causing an inability to control the laser optical power output and wavelength. Further, each of the prior art systems discloses wavelength stabilization techniques for discrete laser sources which are insufficient for certain communication systems demanding higher power optic sources or bundled light sources for higher output power.
The optical amplification in fiber links is a necessity in long-haul communication systems using optical fibers, and also in distribution systems involving large numbers of subscribers. Optical amplifiers enable the optical power in a fiber path to be maintained at sufficiently high levels such that the signal-to-noise ratio (SNR) degradation due to signal shot noise and receiver noise is nearly inconsequential. However, the optical amplifier introduces a dominant noise source to such systems. This dominant noise source, being amplified spontaneous emission (ASE), occurs when the amplifiers are operated at xe2x80x9ctruexe2x80x9d direct current (e.g. microhertz to hertz ranges) and is inversely related to the frequency. Similarly, drift of the electronics and the drift of the laser directly impact performance of the optical systems as longer integration times are required to accurately extract signals from the drift and noise components. It is therefore desirable to filter out noise and drift factors related to the wavelength and system electronics in a communication system.
As reducing output power and utilizing discrete lasers is contrary to the rising communication systems demands, there remains an unsatisfied need for a system of high power light source wavelength stabilization that is more easily and economically fabricated than the prior art systems, which also possesses a convenient and ample output power and wavelength source for achieving future communication demands.
The subjects of the present invention provide methods for stabilizing the output wavelength of an optical laser source, and for reducing the drift in a wavelength stabilized laser source.
It is an object of the present invention to provide a method for stabilizing, to a stability generally in the range of 0.1 ppm to 20 ppm, or better, the wavelength of an array of optical output signals from an optical source array, using a control laser element of the array, wherein the present invention comprises a discriminator, through which a pair of optical beams from the source are propagated; photodetection means for generating an electrical signal in response to each of the beams propagated through the discriminator; a temperature sensing means, for monitoring the temperature of the control laser element; a temperature controlling means, for adjusting the temperature of the control laser element to change the resulting output wavelength; and controlling means for (a) generating an error signal in response to the difference in amplitudes of the two electrical signals generated by the photodetection means, and (b) feeding the error signal to the optical source through the temperature controller to control the output wavelength of the control laser element so as to minimize the amplitude of the error signal.
It is another object of the present invention to provide a method for reducing drift in a wavelength stabilized laser source by frequency modulating with an alternating current a dedicated element of the laser array and filtering noise through a synchronous detection system, having a phase-sensitive detection means.
It is a further object of the present invention to provide a method for utilizing a high power laser diode driver array, comprised of semiconductor or solid-state laser elements, as an optic source for stabilizing the wavelength without reducing output power of the array.
It is still a further object of the present invention to provide a method which can be effectively employed with laser arrays having many individual laser elements, with at least one dedicated laser element of the array generating the optical source to the discriminator.
It is another object of the present invention to provide such a method which is efficient and inexpensive.
As will be more readily appreciated from the detailed description that follows, the present invention offers a number of advantages not previously achieved in the prior art. For example, the present invention offers both wavelength and power stabilization, with the ability to reduce drift in a wavelength stabilized source, in an efficient and inexpensive manner.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with accompanying drawings.